Real-time condition monitoring is a valuable tool for
optimizing reciprocating compressor performance and reducing
unplanned downtime. The ability to detect dangerous failures at
an early stage and bring the compressor to a halt in just two
or three revolutions has saved many machines from serious
damage. For issues that develop more slowly, reliable
monitoring avoids unnecessary work and allows maintenance to be carried out during
planned shutdowns.

A traditional compressor monitoring system uses analog
connections to link sensors on the compressor to an electronic
processing unit located in a safe area. Although many other
areas of the plant will typically use digital communications,
standard remote input/output (I/O) systems and fieldbuses such
as Profibus, Foundation Fieldbus and Modbus are not fast enough
for the real-time vibration monitoring that compressors
require.

This situation has changed, however, with purpose-designed
digital communications available for the most demanding
compressor monitoring applications. Older analog technology is
now obsolete, as digital compressor monitoring has substantial
benefits over the analog technology of the previous
generation.

Compared to traditional analog communications technologies,
digital systems cost less, offer greater flexibility, and are
more reliable thanks to built-in self-checking systems and
resistance to interference. By allowing multiple devices (and
multiple signals from each device) to share a single set of
wires, digital communications cut cabling costs.

In the past, these advantages did not extend to
safety-critical systems, which were still required to be
hard-wired and based on analog technology. This changed around 2008
with the advent of the IEC 61508 standard, which allows the use
of digital systems in applications related to safety.
Experience in the automotive and aircraft industries showed
that properly implemented digital systems provide high levels
of safety protection.

However, even when they are approved for safety-critical
applications, general-purpose digital systems are not fast
enough for real-time vibration and rod position monitoring.
Profibus PA, for instance, has a maximum data rate of 31
kbit/s. This is fine for process control, but too slow for
compressor monitoring, which requires data rates in the Mbit/s
range (approximately 100 times more than Profibus PA).

Analog: Cost and length

Traditional dedicated compressor monitoring systems,
therefore, use analog sensors and cables. Since the compressor
itself is generally mounted in a plant area where there is an
explosive atmosphere risk, the usual arrangement is to wire the
sensors through intrinsically safe (IS) barriers to
rack-mounted monitoring equipment located in a safe area such
as the instrument room (Fig. 1).

Fig.
1. A traditional
compressor monitoring
system uses analog
sensors individually wired
to the control unit.

The field instruments typically installed on reciprocating
compressors are accelerometers and velocity meters for
vibration, indicator pressure transmitters for performance and
rod-load and rod-drop transmitters for rider ring wear and
piston rod runout monitoring. With so many instruments,
implementation costs can be considerable.

There may also be issues with plant layout and electrical
interference. Process plants and compressor stations are full
of electrical noise, and, even with careful shielding, there is
always a risk of interference, which can degrade accuracy and
even cause spurious compressor trips. The risk is greatest with
accelerometers and rod-drop transmitters, which deliver their
output signals in the form of voltages rather than the more
robust 4 mA20 mA current signals used by other field
instruments.

The maximum signal frequency that can be transmitted
reliably depends on the length of the loop, the cable impedance
per unit length, and the ratio of the peak signal voltage to
the current available from the signal conditioner. Integrated
circuit piezoelectric accelerometers provide a high-voltage,
low-impedance output that reduces the effects of long cables
and electrical interference, but even these devices are limited
to a maximum cable length of around 300 m.

Fast digital is the new approach

To overcome the speed disadvantage facing conventional
digital systems, a new type of distributed digital system has
been designed and especially adapted for reciprocating
compressors. The new design allows up to eight analog
transmitters to be connected to an intrinsically safe fast
transmitter interface module (FTIM) located close to the
compressor.

The FTIM pre-processes the sampled data, packages it and
sends it to the systems central interface unit (CIU).
Fig. 2 shows that FTIM and CIU are linked by
industrial Ethernet over a single CAT 7/7A cable.

Fig.
2. Digital
communication requires
only a single Ethernet cable
between the field devices
and the cabinet.

The FTIM gets its power supply directly from the CIU using
power over Ethernet (PoE), as defined by IEEE 802.3af. This
well-proven arrangement avoids the need for separate power
supplies in the hazardous area and takes advantage of the
CIUs diagnostic abilities.

The FTIM continuously monitors every sensor for loss of
signal (sensor unreachable), signals that fall outside the
allowable range and signals that show no variation (stuck or
failed sensor). Similarly, the key hardware components for both
the FTIM and the CIU, and the power supply to the FTIM, are
continuously monitored for malfunctions.

The CIU uses this information to produce a trust
signal confirming the reliability of the system. As soon
as any safety-related failures are detected, the loss of the
trust signal is communicated to the user. The system fulfills
the IEC 61508/61511 (SIL) requirements for machinery protection
and is certified by TÜV Rheinland, Germany.

Digital interface and protocol

Data transmission between the FTIM and the CIU is via a
point-to-point link based on an extended version of the RS-485
interface. Extended means that a special hardware
component is used to boost the transmission range to 500 m
without repeaters. The same component also offers a high level
of protection against electrostatic discharge and ensures
failsafe operation.

The data transmission protocol is based on a digital
interconnect format, as defined in IEC 60958. It supports an
encoding system known as biphase mark code, which combines data
and clock information to produce a self-synchronized data
stream operating up to several Mbit/s.

The actual data flow has two main parts. The CIU first sends
timing information and requests the sampled sensor data, after
which the FTIM sends the corresponding response. Fig.
3 shows how the data stream from the sensors is
divided into packages known as segments. Each compressor
revolution produces several sampled data segments.

Fig.
3. Data is divided into segments
before
being sent to the FTIM. A second
repeat
channel maintains communication in the
event of errors or congestion on the main
channel.

All data sent and received is carried within frames to minimize
data loss and allow any corruption to be detected. To further
increase integrity, each data segment is split across four
frames (Fig. 4). Each frame consists of a
preamble describing the message type, the data itself and a
cyclic redundancy checksum to confirm data integrity. After
every four frames, a marker indicates the end of each
segment.

Fig.
4. Each segment of data is divided
across four frames to increase reliability.

For safety, the data link has two separate transmission
channels (Fig. 5). If the checksum calculated
and sent by the FTIM does not match that calculated by the CIU
after the data has been received, the repeat
channel is activated and the faulty segment is retransmitted at
the same time as the next segment is being sent on the normal
channel.

Data storage and alarms

The CIU stores all the sensor data it receives (both sensor
status information and the actual readings) in a buffer that
holds a minimum of 500 revolutions worth of data.

After initial checking to ensure that the data is complete
and error-free, the systems event manager compares each
signal against two user-configured levels: alarm (lower level)
and safety alarm (higher level), as shown in Fig.
6. To cover different operating conditions, a total of
16 limit sets can be configured for each sensor.

Fig.
6. Built into the CIU is a dynamic
signal
buffer that stores all data from the last
500 revolutions.

For time-wave vibration signals that vary over the course of a
single revolution, each crank angle degree can be automatically
assigned its own alarm level, with up to 360 separate values
per rotation. This allows the system to detect dangerous
vibrations reliably during quiet parts of the cycle, without
being swamped by high vibration levels during noisy events such
as valve movement and rod load reversal.

Fig. 7 shows the actual vibration reading
over one complete revolution and the envelope of the vibration
readings since the last machine overhaul. In addition, the
so-called persistence envelope displays the
vibration readings of several previous revolutions for better
visualization of this highly dynamic signal. With such
high-resolution alarm levels, emerging problems can be detected
immediately and the compressor shut down before major damage
can occur. This is not possible with conventional monitoring
systems.

Fig.
7. Time-wave vibration signal over
one
revolution with alarm limits referenced to
the
crank angle.

In the event of a limit violation resulting in an alarm or a
safety alarm, the CIU copies the sensor data from the latest
500 revolutions into a separate event buffer. The maintenance supervisor or
reliability engineer can download the contents of the event
buffer at any time for offline analysis.

The digital advantage

Performance monitoring and machinery protection systems
allow the compressor condition to be assessed while it runs,
supporting maintenance decisions and protecting valuable
assets. Repair work can be better planned, and spare parts
organized in good time. Most importantly, condition monitoring
can cut downtime and increase production.

Digital compressor condition monitoring systems have many
advantages over their analog predecessors. During design,
installation and commissioning, they offer: